Solar cells are key elements in photovoltaic technologies that convert solar light directly into electricity, and are widely used in a variety of applications from the universe to homes.
A solar cell is basically a diode having a p-n junction and its operation principle is as follows. When solar light having an energy greater than the band gap energy of a semiconductor is incident on the p-n junction of a solar cell, electron-hole pairs are generated. By an electric field created at the p-n junction, the electrons are transferred to the n layer, while the holes are transferred to the p layer, thereby generating photovoltaic force between the p and n layers. When both ends of the solar cell are connected to a load or a system, electric power is produced as current flows.
Solar cells are classified into a variety of types depending on the materials used to form an intrinsic layer (that is, light absorption layer). In general, silicon solar cells having intrinsic layers made of silicon are the most popular ones. There are two types of silicon solar cells: substrate-type (monocrystalline or polycrystalline) solar cells and thin film type (amorphous or polycrystalline) solar cells. Besides these two types of solar cells, there are CdTe or CIS (CuInSe2) compound thin film solar cells, solar cells based on III-V family materials, dye-sensitized solar cells, organic solar cells, and so on.
Monocrystalline silicon substrate-type solar cells have remarkably high conversion efficiency compared to other types of solar cells, but have a fatal weakness in that their manufacturing costs are very high due to the use of monocrystalline silicon wafers. Also, polycrystalline silicon substrate-type solar cells can be produced at relatively low manufacturing costs compared to monocrystalline silicon substrate-type solar cells, but they are not much different from monocrystalline silicon substrate-type solar cells because solar cells of both types are made out of bulk raw materials. Thus, their raw material price is expensive and their manufacturing process is complicate, thus making it difficult to cut down the manufacturing costs.
As one solution to resolve the deficiencies of those substrate-type solar cells, thin film type silicon solar cells have drawn a lot of attentions mainly because their manufacturing costs are remarkably low by depositing a silicon thin film as an intrinsic layer over a substrate such as glass. In effect, the thin film silicon solar cell can be produced about 100 times thinner than the substrate-type silicon solar cell.
Amorphous silicon thin film solar cells were firstly developed out of the thin film silicon solar cells and are started to be used in homes. Since amorphous silicon can be formed by chemical vapor deposition (CVD), it greatly contributes for mass-production of amorphous silicon solar cells and low manufacturing costs. On the contrary, amorphous silicon solar cells have very low photovoltaic efficiency compared with substrate silicon solar cells because of a lot of silicon atoms with dangling bonds present in amorphous silicon. In addition, amorphous silicon solar cells have a relatively short lifespan and their efficiency is more apt to degrade with increased use thereof.
Therefore, as an effort to complement the foregoing shortcomings of amorphous silicon (thin film) solar cells, polycrystalline silicon thin film solar cells and tandem thin film solar cells having at least two photovoltaic units have been developed.